The frequency assignment in the Personal Digital Cellular (PDC) scheme by which services are currently provided in Japan can be operated by the following two methods. One is the Fixed Channel Assignment (FCA) scheme. This is a scheme in which propagation characteristics and traffic distributions are estimated in advance using actual measurements or theoretical calculations, and frequencies are arranged to each base station fixedly such that a required CIR for frequency assignment is satisfied at an area edge.
Another one is the Dynamic Channel Assignment (DCA) scheme. This is a scheme in which a common frequency that is available to all base stations is provided in the system, and this frequency is assigned in the case where there is no unused frequencies for the FCA. Unlike the FCA, the DCA is a scheme for assigning frequencies adaptively according to a state of frequency utilization in the surrounding at a time of assignment.
As described, the FCA arranges frequencies on an assumption that a frequency will be assigned to a mobile station at an area edge, whereas the DCA judges whether an assignment is possible or not according to the identical frequency interference condition at the base station and the mobile station at a time of assignment in general so that it is expected to have a good frequency reuse efficiency but it has a problem in that a communication quality can be largely affected in conjunction with a change of the radio signal propagation environment depending on a moving direction of the mobile station after the assignment.
For example, a mobile station to which a frequency is assigned by the DCA immediately below the base station will have a high receiving level for desired signals from the base station, so that the assignment is possible even when the identical frequency interference level is high. However, if this mobile station moves away from the base station after the assignment, the receiving level of the desired signals will obviously decrease so that there is a possibility for becoming more easily affected by the identical frequency interference.
On the other hand, in the case of carrying out the radio channel assignment by the autonomous distributed type DCA in the TDMA scheme, there is a need for a mobile station of the second call in the identical radio carrier to measure the interference level by specifying a slot, but it is difficult to measure the interference level of the assignment candidate radio channel before establishing synchronization with the base station, so that the assignment is made without measuring the interference level in general. For this reason, it is currently customary to adopt the operation in which a margin with respect to the required CIR (Carrier-to-Interference Ratio) for assignment is set large at a time of assigning the first call such that the mobile stations of the second and subsequent calls can be connected even under the worst condition.
As described, in the conventional TDMA mobile communication scheme, the radio channel is assigned by setting a margin with respect to the required CIR for assignment large at a time of assigning the radio channel to the first call such that the mobile stations of the second and subsequent calls can be connected even under the worst condition, rather than measuring the interference level with respect to the mobile station of the second call in the identical radio carrier, so that there is a problem in that the frequency utilization efficiency is degraded.